Data of longitudinal magneto-optical Kerr effect (LMOKE) probing of demagnetization following optical (Opt) and terahertz (THz) excitations in 4 nm iron film on MgO substrate. Pump and probe incidence angle is 50 deg, fluence is on the order of 0.1 mJ/cm^2, time step is 20 fs. In each case both rotation (Rot) and ellipticity (Elp) variations are recorded. Demagnetization ("Demg") traces are the signals odd in sample magnetization and even in the driving field in case of THz pump. These traces are normalized by the static LMOKE. Zeeman response ("Zeem") is odd in both magnetization and driving THz field. Measured traces are deconvoluted with the pump profile, which in case of THz pump is defined as derivative of the Zeeman response and in case of optical pump is measured through Kerr effect in diamond. "True" M quenching is calculated as linear combination of Rot and Elp demagnetization traces for both optical and THz excitations.

We acknowledge funding by the German Science Foundation through the collaborative research center SFB TRR 227 "Ultrafast spin dynamics" (projects A01, A05, B02 and B03)